According to the fundamental invention of Segal (SU-A-946830 and M. G. Segal: “Ways of Numerical Program Control Utilization in Machine Tools for Machining Round Teeth of Conical and Hypoid Transmissions,” Izvestiya vuzov, Mashinostroenie, 1985, No. 6, pp. 120–124), a conventional mechanical generating machine for the production of spiral bevel gears, having a tiltable cutter head on a cradle and up to ten axes, of which a maximum of three axes rotate dependent on one another during the generating cutting operation, can be replaced with a numerically controlled generating machine with six axes at the most, all of which work simultaneously and in accordance with non-linear principles, to produce the same bevel gears as on the conventional machine.
Since then, a series of CNC-controlled bevel gear machines representing different embodiments of the principle introduced by Segal have been made public, for example a multi-axis bevel and hypoid gear generating machine according to EP-B1-0 374 139. What all CNC-controlled machines with 6 axes have in common is that a work gear spindle support has to be continuously repivoted about a vertical axis during the generating motion. This is referred to as machine root angle pivoting. This machine root angle pivoting is necessary because the cutter head, which on a conventional mechanical generating machine moves on a curved cone surface in the case of a conical generating gear, is moved by a cross slide in one plane on a numerically controlled generating machine. However, in a machine according to WO 02/066193 A1, the tool spindle and the work gear spindle have been interchanged, so that there the tool spindle support has to be repivoted. This, however, does not alter anything in the principle set forth by the fundamental invention of Segal.
The machine root angle pivoting of the work gear spindle support generally reverses its direction within one generating cutting operation. On the one hand, this entails a control problem, particularly in the single indexing method (face milling), in which the machine root angle pivoting takes place at a greater speed than in the continuous indexing method (face hobbing). On the other hand, this entails a load reversal in those mechanical elements which effect the movement of the pivotable work gear spindle support. Even if there is no backlash in the drive train for the machine root angle pivoting, because only pre-stressed bearings, joints and rotary spindles are used, these are elastic machine elements which alter their deformation in a load reversal and thereby cause inaccuracies in the bevel gear tooth surfaces produced. The greater the radius of action of the pivotable spindle support, the more critical this effect becomes, since small errors are then magnified due to the motion reversal. The required pivotability of the work gear spindle carrier during machining impairs its stiffness. However, for the production of bevel gears with precisely specified tooth flanks the greatest stiffness is actually imperative.
DD-A1-255296 discloses a gear-cutting machine for the production of bevel gears in generating and profile grinding methods, in which the work gear spindle support can be pivoted about a vertical axis and the work gear and tool spindle supports are vertically and horizontally displaceable relative to one another. The purpose of this is to substantially reduce the expenditure for the gear-cutting machine and its drive and control technology and to increase its degree of automation. This known machine with 8 axes essentially differs from a mechanical cradle machine only in that the cradle, on which an adjustable swivel drum with the tool is eccentrically mounted, is replaced with a cross slide by means of which the swivel drum bearing the tool can be displaced in two coordinates arranged at right angles to one another. The tool axis forms an adjustable tilt angle with the swivel drum axis. On a classic cradle machine the orientation of the tilt angle is fixed by an additional angle (swivel). As opposed to this, in a cross-slide machine the orientation of the tool inclination must additionally be slaved to the original cradle rotation by continuously rotating the swivel drum. For each type of bevel gear to be produced the tilt angle has to be set anew on the known machine. For this purpose, a tool tilting mechanism known from classic cradle machines is arranged between the swivel drum and the tool. During machining a bevel gear series, this tilting mechanism is clamped tight and the orientation taken into account with the initial position of the swivel drum rotation remaining unchanged. Whenever a different bevel gear is to be produced, a different tilt angle between the tool axis and the swivel drum axis must be adjusted with the aid of the tilt mechanism, and a different initial position of the swivel drum must be provided with the aid of the control. A further adjustment relates to the work gear spindle support. The machine root angle, to which the work gear spindle support is pivoted and which remains fixed during machining, is the work gear setting angle δE. In the generating method this angle depends on the pitch angle of the bevel gear to be produced. In the profile grinding method the gear setting angle δE depends primarily on the shaft angle of the bevel or hypoid gearing. The machine root angle as well is set anew in every new type of bevel gear to be produced. The rolling operation then runs as on the classic mechanical cradle machine. The only difference is that to move the tool the swivel drum is not pivoted by the cradle around the cradle axis, but rather by a separate rotary drive.
A multi-axis generating machine with CNC control for the production of bevel and hypoid gears is known from EP-B1-0 374 139 already mentioned above. In this known machine the tool spindle is moved in a vertical plane by a cross slide. Here, the axis of the tool spindle remains constantly horizontal. At the same time, during the generating cutting the work gear spindle has to be pivoted about a vertical pivot axis. This known machine is a so-called 6-axis machine. The six axes include three translationally movable axes and three rotational axes. One of the three rotational axes is the vertical pivot axis. All six axes are controlled simultaneously during generating cutting. The pivoting movement of the work gear axis about the vertical pivot axis generally has a reversal point. This means that the pivoting movement slows down as it approaches a maximum machine root angle, then reverses its direction and speeds up again. As already explained above, a pivoting movement of this nature entails control problems, since the movement must be braked and then accelerated again in the opposite direction. The load reversal also already mentioned above, which causes the form changes in the drive train of the pivoting mechanism, can result in undesired deviations on the flanks of the bevel gears produced.
In the machine known from the above-mentioned DD-A1-255296, the tilting mechanism, which has to be set a new for every new type of bevel gear to be machined, causes precision problems. Furthermore, providing the tilting mechanism on a swivel drum is very complicated. To be sure, the machine according to EP-B1-0 374 139 does not have this disadvantage. However, it does have the disadvantage described above, which results from the necessary pivoting movement of the work gear spindle about the vertical pivot axis.
It is the object of the invention to design a machine and a method for CNC-controlled machining, particularly generating cutting or generating grinding, of spiral bevel gears with and without hypoid offset, in such a manner that no adjustable tilting mechanism is required for the tool axis, but nevertheless, no machine root angle pivoting of one of the spindle supports is required either.
This object is achieved in accordance with the invention in the machine and the method, with the features and steps given in claims 1 and 22, respectively.